Rising levels of carbon dioxide (CO2) in the Earth's atmosphere, caused primarily by combustion of fossil fuels, have prompted concern that temperatures at the Earth's surface will increase sharply during the 21st century. To address this issue, numerous nations are developing plans for lowering CO2 emissions to the atmosphere. The principal approaches under consideration are: improving energy efficiency; making greater use of alternative sources of energy; and developing economically viable technologies for capture, separation, and long-term storage of CO2. The latter strategy, known as “CO2 sequestration,” is receiving increasing attention because it permits continued use of high-carbon fossil fuels to generate electrical power while ensuring that CO2 releases to the atmosphere are reduced.
A potentially attractive means for CO2 sequestration is injection of gaseous CO2 into underground reservoirs, e.g., active or depleted oil and gas fields, deep brine formations, and subterranean coalbeds. The underlying premise of this approach is that, after injection, the CO2 will remain sequestered in the host rock for hundreds, perhaps even thousands, of years. In practice, however, such long-term reservoir integrity cannot be guaranteed. If either gaseous CO2 or CO2-saturated formation water escapes to overlying strata or to the surface, underground and subaerial water supplies could become seriously contaminated, and/or large amounts of CO2 could be released to the atmosphere.
Significantly, the reservoir-integrity problems associated with subterranean sequestration of gaseous or liquid CO2 can be completely avoided by chemically binding CO2 with suitable solid materials. This alternative CO2 sequestration strategy, known as “mineral carbonation,” involves reaction of CO2 with naturally occurring silicates to produce solid carbonate compounds, such as calcite (CaCO3) and magnesite (MgCO3), for the purpose of long-term terrestrial isolation of CO2. “Mineral carbonation” also implies a chemical process carried out at elevated temperatures and pressures in an industrial-scale reactor, because a similar term, “mineral trapping,” alludes to crystallization of carbonate compounds at ambient temperature and pressure after CO2 is injected into a subsurface geologic formation. The U.S. Department of Energy (DOE) classifies mineral carbonation as a “CO2 conversion” technology, rather than a geological CO2-sequestration strategy, because in mineral carbonation most, if not all, of the CO2 is converted to one or more solid carbonate compounds, whereas in mineral trapping only a tiny fraction (generally less than one volume %) of the injected CO2 is ultimately incorporated into solid carbonates.
Mineral carbonation has many important advantages over alternative methods for large-scale CO2 sequestration. First, the carbonate compounds formed in the process are thermodynamically stable, environmentally benign, and weakly soluble in meteoric water. Consequently, they can be amended to soils to reduce acidity and increase moisture content, combined with stone to strengthen roadbeds, or simply dumped in a landfill. Alternatively, the carbonates could be returned to the site of excavation to fill the cavity created by soil/rock removal. Regardless of the particular end use or disposal scheme selected for the carbonates, the reacted CO2 will remain tightly bound in the crystallographic structures of the carbonates, immobilized for an indefinite period of time. Therefore, a commercial mineral carbonation technology creates no major “legacy issues” for nearby population centers. In contrast, other proposed methods for wide-scale CO2 sequestration, such as subsurface storage in brine formations, and disposal in deep-ocean realms, rely on risky environmental factors to ensure long-term CO2 containment: an impervious, superjacent “caprock” in the case of subsurface injection of CO2 into brine formations, and low ambient temperature and high ambient pressure, with no current-driven dispersal of the sequestration “agent” (liquid CO2 or CO2-hydrate), in the case of suboceanic CO2 disposal.
In weighing the technical feasibility of CO2 sequestration by mineral carbonation, it should be noted that huge masses of rocks and clay-rich formations suitable for carbonation occur worldwide. For example, ultramafic complexes and large serpentinite bodies are major sources of the magnesium-rich minerals olivine (forsterite) and serpentine, which can be carbonated by the reactions
Moreover, contact-metamorphosed limestones frequently contain wollastonite (CaSiO3), and large quantities of plagioclase [(Cax, Na1−x)(Al1+xSi3−x)O8] are present in many different types of common rocks. Wollastonite and plagioclase can be converted to calcite (plus silicious solid material) by the reactions

Another key attribute of mineral carbonation, in relation to other technologies that deal with CO2 waste streams, is that costs associated with CO2 transport are potentially very low. This is so because in an industrial-scale implementation of a mineral carbonation technology, the metal-silicate feedstock can be carbonated in commercial facilities located adjacent to, or near, large “point sources” of CO2 generation, such as fossil fuel-fired power plants, cement factories, and steel mills. In contrast, CO2 sequestration in deep brine aquifers, or the benthic regions of the world's major oceans, would often require CO2 transport over substantial distances. Building and maintaining many miles of pipeline to achieve such transport, or hauling liquid CO2 over long distances by truck, train or ship, would be extremely expensive and perhaps totally impractical.
Finally, the following additional advantages of mineral carbonation are noteworthy: (1) by technical necessity, mineral carbonation involves rapid conversion of CO2 to solid carbonate(s), and (2) by virtue of creating one or more solid carbonate phases from a volatile phase rich in CO2, carbonate crystallization automatically produces a large reduction in total volume. It has already been demonstrated by researchers at the Albany Research Center in Oregon, and the Los Alamos National Laboratory in New Mexico, that, with vigorous mechanical stirring, olivine and heat-pretreated serpentine can be quantitatively converted to magnesite (see Reactions 1 and 2 above) in ˜30 minutes at 155° C. and 185 atm total (fluid) pressure. Significantly, the latter processing conditions are readily attained in modem industrial reactors. A large reduction in the total volume of the reactants (CO2, plus one or more condensed phases, and often one or more “additives” and/or catalysts) is automatically achieved in mineral carbonation because the CO2-bearing solids produced are >1000× more dense than gaseous CO2 at STP (standard temperature and pressure: 25° C., 1 atm). This contraction essentially eliminates the “room problem” associated with storing large volumes of CO2 (as a gas, liquid or supercritical fluid) in subsurface rock formations.
While it is evident that mineral carbonation offers many important advantages over competing CO2 sequestration technologies, it is also true that it suffers two major disadvantages. Chief among these is the need to mine, or quarry, large quantities of silicate feedstock to sequester the gigatons of atmospheric CO2 generated annually by combustion of fossil fuels. Excavating massive amounts of rock and soil to permit silicate carbonation at sites near major industrial sources of CO2 will be expensive, and will require intense reclamation activities to restore the land to an environmentally acceptable state. However, there is no doubt that this can be accomplished using modern methods of environmental restoration. In addition, it is likely that new technologies will soon be developed to enable innovative synergies, and more satisfactory compromises, between large-scale energy production and traditional modes of land use.
The second major disadvantage of mineral carbonation is that elevated temperatures and pressures, and chemical “additives” and/or catalysts, are usually required to accelerate CO2 conversion to one or more crystalline carbonates. While considerable success has already been achieved in carbonating olivine (Reaction 1) at commercially feasible temperatures and pressures, mineral carbonation experiments performed over the past four years at the Albany Research Center have shown that untreated serpentine does not react as readily (Reaction 2). To date, the only known remedy for sluggish serpentine carbonation is to heat-pretreat the mineral to 600-650° C. prior to carbonation, which drives off structurally bound water (hydroxyl groups). Tests of this altered (dehydroxylated) serpentine have shown that it is much more reactive than untreated (hydroxylated) serpentine. However, at a typical fossil fuel-fired power plant, heat treating serpentine at 600-650° C. prior to carbonation would require ˜200 kW·hr of electricity per ton of serpentine feedstock. With one ton of carbon in a fossil fuel producing ˜3.7 tons of CO2, and each ton of CO2 consuming ˜2.0 tons of serpentine during carbonation, the power requirements for serpentine dehydroxylation represent 20-30% of total power output. This large energy penalty threatens the economic viability of CO2 sequestration by serpentine carbonation.
It is evident from Reactions 3 and 4 that the problems plaguing serpentine carbonation would be partly or entirely avoided if a more abundant silicate mineral could be utilized. In this regard, it is noteworthy that wollastonite is carbonated by Reaction 3 at 60° C. using an aqueous solution of acetic acid as a catalyst. This result is of some scientific interest, but it fails to significantly bolster metal-silicate carbonation as a potential means for sequestering large masses of CO2 because wollastonite, while not rare in nature, is typically found in significant quantities only in contact metamorphic aureoles where it tends, along with other silicates, to form small, isolated bodies adjacent to igneous intrusions. The other principal occurrence of wollastonite is as a widely disseminated mineral in regionally metamorphosed carbonate strata. Thus, wollastonite is not available in sufficient quantities to sustain a wide-scale silicate carbonation technology.
The low abundance of wollastonite leaves plagioclase as the major potential source of calcium (Reaction 4) to produce the quantities of carbonate required to sequester gigatons of CO2 by metal-silicate carbonation. (Other, locally significant potential sources of calcium include Ca-rich clay deposits, Ca-rich fly ash, and waste concrete.) However, a commercially feasible plagioclase carbonation technology faces two formidable technical challenges. First, it is inherently difficult to extract calcium from plagioclase because, being a framework silicate with a three-dimensional structure held together by tightly bonded atoms of silicon and aluminum, plagioclase is not readily destabilized by firing at high temperatures, or easily “digested” (decomposed) by most customary solvents. Second, while most plagioclases contain a significant amount of calcium, Ca-contents are always less than that of wollastonite. Therefore, per ton of silicate feedstock, less calcium-rich carbonate (calcite) is formed from plagioclase than from wollastonite. These difficulties notwithstanding, it is clear that plagioclase carbonation merits serious scientific study to determine whether it could be an attractive alternative to serpentine carbonation in sequestering large quantities of CO2.